1. Field of Invention
The present invention is directed to scanning probe microscopy. More particularly, the present invention is directed to a method and apparatus for characterizing and manipulating a sample.
2. Description of Related Art
Presently, scanning probe microscopes (SPMs) are typically used to determine the surface characteristics of a sample, commonly biological or semiconductor samples, to a high degree of accuracy, down to the Angstrom scale. An SPM operates by scanning a measuring probe assembly having a sharp stylus over a sample surface while measuring one or more properties of the surface. One example of an SPM is an atomic force microscope (AFM) wherein a measuring probe assembly includes a sharp stylus attached to a flexible cantilever. Commonly, an actuator such as a piezoelectric tube, often referred to as a piezo tube, is used to generate relative motion between the measuring probe and the sample surface. A piezoelectric tube moves in one or more directions when voltages are applied to electrodes disposed inside and outside the tube.
In the operation of an AFM, preferably, a measuring probe assembly is attached to a piezoelectric tube actuator so that the probe may be scanned over a sample fixed to a support. According to an alternative method, the probe assembly is held in place and the sample, which is coupled to a piezoelectric tube actuator, is scanned under it. In both AFM examples, the deflection of the cantilever is measured by reflecting a laser beam off the backside of the cantilever and towards a position sensitive detector.
In a contact mode or deflection mode of AFM operation, the AFM operates by placing the tip at the end of the cantilever of the probe directly on a sample surface so that the cantilever obtains a preset deflection. Preferably, the force between the tip and the sample surface is selected by the user, and defines an operating point of the AFM, the deflection setpoint. When scanning the surface laterally, the response of the cantilever to variations in the surface is monitored by an AFM deflection detection system and can be used to create an image of the sample surface. As suggested previously, a typical deflection detection system employed in AFM is an optical beam system that optically determines the deflection of the cantilever by light reflected off the cantilever onto a detector. Often, the height from the surface of the sample to a fixed end of the cantilever is adjusted with feedback signals that operate to maintain a predetermined amount of cantilever deflection (i.e., generally at the deflection setpoint) during scanning. A reference signal is often applied to one input of a feedback loop of an AFM system. The output of the feedback loop is then applied as an adjustment signal to an actuator to maintain constant cantilever reaction and maintain relative height. An image of the surface is then created by monitoring the feedback signals and plotting the adjustment amount versus lateral position of the cantilever tip.
In TappingMode™ (which is a trademark of Veeco Instruments, Inc.), a probe tip makes contact with a sample as it taps across the surface of the sample. An AFM employing TappingMode™ uses oscillation of a cantilever to reduce the forces on the sample. In particular, a specific cantilever is oscillated near or at its resonant frequency. A feedback loop is used to maintain a desired amplitude of oscillation. The feedback circuit adjusts a vertical position of the cantilever or the sample to maintain the desired amplitude as the cantilever traverses the surface of the sample. The signals used to adjust the vertical position are used to create an image of the surface versus the lateral position of the cantilever tip.
Unfortunately, present SPMs do not provide an adequate mechanism for manipulating the sample being imaged. For example, present SPMs do not adequately provide for accurately picking up, nudging, etc. the sample or portions thereof. Manipulation of the sample can be difficult because present SPMs do not allow for performing action on what often times are nanoscale objects. Present SPMs typically use the same probe to manipulate and image or they use one probe to manipulate and one probe to image but the design is such that there is a large horizontal distance between the manipulating and the imaging probes. This can create inaccuracies in determining the location of the manipulation probe with respect to an imaging probe because inaccuracies in offset determinations and inaccuracies in the precision of moving the probes. This offset can result in errors when attempting to manipulate the sample. Furthermore, present SPMs do not provide for adequate accuracy in manipulating particles on a sample due to limitations in the control accuracy of the actuator of the SPM. Another difficulty is that present SPMs encounter drift in the relative positions of the tip and sample, which can result in a probe manipulating an incorrect position on the sample. Therefore, an SPM that overcomes these drawbacks was desired.